(1) Field of the Invention
The present invention relates to a twist drill suitable for use in a boring of a hole without using cutting oil or in that while spraying a very small quantity of cutting oil onto frank faces of a tip end portion of the twist drill. More particularly, the present invention relates to a twist drill which is made of a high-speed steel or a cemented carbide. Further, the present invention relates to a twist drill in that at least on the tip end portion of the twist drill is coated with a Ti-based hard material film comprising at least one layer of TiN, TiC, TiCN, TiAlN or TiAlNC.
(2) Description of the Related Art
One example of a conventional twist drill is shown in FIGS. 3(a) to (d) and FIG. 4. As seen in FIG. 3(a), the prior art twist drill has a drill body 30 rotatable about an axis in a drill rotation direction (arrow A), a pair of chip discharge flutes 16 formed in the outer peripheral surface of the drill body 30, a pair of chisel edges 11 formed on a tip end of the drill body 30, a pair of inner cutting edges 12 extending from each outer end of the chisel edges 11 and a pair of major cutting edges 13 extending from each outer end of the inner cutting edges 12 forming substantially straight lines. Also from FIGS. 3(c) and (d), a pair of the major cutting edges 13 are configured as ridge lines formed by both the flutes 16 and flank faces or flanks 20 having an angle .alpha.11 and a relief angle of .alpha.12. As seen in the cross-sectional view shown in FIG. 4, each flute 16 exhibits a configuration formed by one curve or a combination of a plurality of curves having a point angle .alpha.11. Since each major cutting edge 13 is formed as a substantially straight line, the leading edge 17 of the land 19 formed by both the land 19 and the flute 16 has a positive rake angle of .alpha.13. Further, each leading edge 17 forms an acute angle of .alpha.14.
Generally, in a boring of a hole using cutting oil, wear or chippings of the cutting edges are reduced by a cooling effect caused by the cutting oil. However, in a boring of a hole without using cutting oil or in that while spraying a very small quantity of cutting oil onto frank faces of a tip end portion of a twist drill, since no cooling effect is caused by the cutting oil, the wear or the chipping of each cutting edge occurs. As seen in the cross-sectional view shown in FIG. 4, when each leading edge 17 of the land 19 has an acute angle of .alpha.14, the outer comer 15 formed by the flute 16, flank 20 and the land 19 has a sharp configuration and especially the wear or the chipping rapidly occurs at the outer corner 15 of the major cutting edge 13 where the cutting speed is maximum. When using cutting oil, chips or machined material generated by the boring of the hole are transferred and smoothly discharged out through the spaces formed between the bored hole and the chip discharge flutes by the aid of the lubricant effect of the cutting oil. However, in a boring of a hole without using cutting oil, no such lubricant effect is effected. Thereby, a large frictional resistance occurs, resulting in chippings of the leading edge 17 of the land 19 occurring. That is, when the leading edge 17 of the land 19 has a sharp configuration or a positive angle, the leading edge 17 is prone to chipping.